Inexpensive inertial navigation systems that are aided by only platform relative horizontal position change or velocity references may not undergo appropriate motions to observe heading drift rate. For example, when a gyroscope in a navigation system of an airborne vehicle is oriented to be sensitive to an axis that is perpendicular to a plane, which is tangential to the point on the earth directly below the gyroscope, and if the altitude of the airborne vehicle is not significantly varying, the heading drift of the gyroscope cannot be corrected by the filters in the inertial navigation system. Over time, the heading drift of the gyroscope results in large position errors in the inertial navigation system. This heading drift can cause the airborne vehicle to go off course.